Portable telephones and more particularly cellular telephones are now widely used because of the convenience and mobility that such devices offer. The extensive build up of cellular telephone service coverage has contributed to the wide use of such devices.
As subscribers of wire line telephone service make the transition to cellular service, they expect the same levels of features and functionality in the cellular product as they receive from their wire line products. In the past, often times this was not feasible because of the size, power and cost constraints place on the designers of cellular telephone products.
Cellular telephones are traditionally designed for one of three basic product categories; mobile, transportable, and portable. Mobile products are typically mounted in a motor vehicle and draw their power for the vehicle power supply. Mobile products are characterized as relatively large, and as having a virtually unlimited amount of electrical current available. Trans-portable products are similar to mobile products, but they can be carried by the subscriber. Trans-portable products comprise a large capacity battery and generally offer the same features and functions that a mobile product offers.
Potable products differ from the other categories in that they are much smaller and are designed to be easily carried on the person of the subscriber. Yet, subscribers still desire to have a full compliment of features as would be available in a wired, mobile, or trans-portable telephone.
it is common for wired telephones to incorporate a loud speaker so the subscriber can monitor the call progress before placing the headset next to their ear. Alternatively, the loud speaker may be connected to a circuit including the microphone so that the telephone may be operated in a hands-free mode wherein the user can both speak and listen in a conversation without holding the hand set near their head. The use of a loud speaker is especially useful and desirable as more and more services such as voice mail and automated attendants are employed. When these type of services are employed, the subscriber may spend a substantial portion of a conversation listening as opposed to conversing with another individual. In such a circumstance, there is no need to hold the head-set positioned so that the microphone is in close proximity to the subscriber's mouth. In fact, not holding the head-set frees the subscriber's hands for note taking or other activities.
Incorporating a monitor feature in a portable cellular telephone has been difficult because of several constraints and certain operational problems involved with designing such a product. Among these are the necessarily close proximity between the loud speaker and the microphone and the potential for feed-back in such a design. Also, the limited space for controls to operate such a feature and the need to make the feature easy no operate and configure have prevented any manufacturers from offering such a feature in a portable cellular telephone.